1. Field of the Invention
This invention pertains generally to power distribution systems and, more particularly, to such systems including switchgear cabinetry, a circuit breaker and an arc hood. The invention also relates to an arc hood.
2. Background Information
Low-voltage power circuit breakers with insulated housings typically have arc chute vents on the top of the housing. During overload and short circuit conditions, arc exhaust gas is expelled from the vents. The exhaust gas can be extremely hot and ionized and may carry metal vapor. Furthermore, the exhaust gas can carry stray current from the arc in the circuit breaker to grounded metal features in a sheet metal switchgear enclosure, thereby exceeding the limits allowed for ground current flow. The exhaust gas is also expelled with explosive force and may easily damage parts of that enclosure.
Switchgear cabinetry is typically designed to include one or more channels into which arc gases can be directed for dissipation thereof. In this regard, such switchgear cabinets typically include an arc hood that is mounted within each cell and disposed above the vents in the circuit breaker through which the arc gases are exhausted.
U.S. Pat. No. 6,388,867 discloses an arc hood mounted on a pair of spaced sidewalls of a circuit breaker cassette with a pair of brackets. The arc hood is positioned to extend over and be disposed at least partially adjacent the circuit breaker when the circuit breaker is in a racked position. The arc hood is of a generally upside down U-shape in cross section and thus provides a flow channel that is open at the opposite ends thereof. The circuit breaker is configured with a plurality of vents at the upper end thereof through which the arc gases are discharged. The arc gases are preferably discharged out of the immediate vicinity of the circuit breaker to avoid direct contact between the highly-ionized arc gases and the line contacts at the rear of the circuit breaker. The arc hood with its flow channel is thus provided to direct the arc gases flowing out of the vents in a sideways discharge direction and toward various regions of the switchgear cabinet where the arc gases can cool and become deionized. Since the arc gases are highly pressurized, at least a nominal portion of the arc gases undesirably flows or leaks through a gap in a leakage direction. A seal apparatus advantageously resists the flow of arc gases in the leakage direction. The seal apparatus includes a generally planar seal member and a plurality of fasteners. The seal member is a flexible strap that is manufactured out of an elostomeric material or other appropriate material that is suited to withstand the high temperatures and high pressures of the exhaust gases that are produced by the circuit breaker.
Typically, an insulated barrier is placed somewhere above the arc chute exit to manage the effects of the arc exhaust. At relatively high currents (e.g., over 100,000 A), however, the volume of exhaust gas, and the magnitude of the pressure wave from interruption, may damage the typical barrier and fill the enclosure with enough gas to conduct excessive currents to the various grounded metal features. The exhaust gas can even fill the area of the circuit breaker primary connections and trigger spontaneous arcs among the conductors. An improved arc hood is, therefore, needed which can contain and diffuse the damaging pressure wave while cooling and de-ionizing the exhaust gas before allowing it into the general enclosure space.
Relatively higher voltage applications (e.g., up to 1000 V and higher) of power circuit breakers also encounter arcs among the conductors and to the enclosure ground due to arcing exhaust gas, even at modest levels of overload and short circuit current. An improved arc hood is needed that requires less auxiliary insulation in the enclosure and allows new applications for low-voltage and relatively higher voltage circuit breakers without arc exhaust gas difficulties.
Accordingly, there is room for improvement in arc hoods and in power distribution systems employing the same.